The damaging effect of conventional detergents used to wash the body surface, particularly where young, tender or damaged skin is involved, has been the subject of intense study for many years in a search for milder-to-the-skin products, which not only cleanse the skin efficiently, but also leave the skin with a pleasant smooth silky feel after the skin surface has been dried off.
The use of certain mono- and di-alkyl phosphate salts for this purpose has been advocated in view of their mild characteristics, but some of this group of salts are used as antifoam agents because of their lather suppressant properties, and would therefore require careful formulation if lather control is not required.
To this end, U.S. Pat. No. 4,139,485 (Kao Soap Co. Ltd.) describes a detergent composition having low irritation properties on human skin, wherein the surfactant component is dialkyl or dialkenyl phosphate salt (DAP) and/or monoalkyl or monoalkenyl phosphate salt (MAP), each alkyl or alkenyl group having from 10 to 16 carbon atoms), the weight ratio of `DAP` to `MAP` being from 20:80 to 0:100. This system is stated to possess good detergency.
Also, in U.S. Pat. No. 4,526,710 Kao Corporation report a study to improve properties of detergent composition which make use of anionic phosphate surface active agents, which Kao maintain are highly innoxious and particularly mild to the skin. As a result, Kao have found that when phosphate ester salts having a specific ion pair, notably mono- or di-alkyl (C8-18) phosphates, are used in combination with alkanol amine salts of higher fatty acids and alkyl amine oxides, the detergency and foaming characteristics are remarkably improved.
Also, U.S. Pat. No. 4,758,376 (Kao) discloses an alternative composition, comprising an alkanolamine salt of a mono or dialkyl (C8-18) phosphate or mixtures thereof, to that described in U.S. Pat. No. 4,526,710 in which the problem of poor foaming due to the dialkyl phosphate salt is dealt with by incorporating with the phosphate a compound chosen from an amidoamine amphoteric surfactant or hydroxysulphobetaine, or an aliphatic lactylate or glycolate.
It is clear from a study of the three Kao references, that all the evidence on which their disclosures are based is restricted to alkyl phosphates or dialkyl phosphates in which the alkyl group has at least 10 carbon atoms. There is absolutely no reference at all to any evaluation of an alkyl phosphate ester or dialkyl phosphate ester salt where the alkyl group or groups contains only 9 carbon atoms, or less.
Applicants in their search for a mild surfactant for use in cleansing human skin or hair, with the added attribute that a full, soft lather is produced without necessarily incorporating a secondary surfactant such as those proposed by Kao, have unexpectedly discovered that a narrow range of certain dioctyl phosphate salts meet their requirements in this respect. Furthermore, closely related dialkyl phosphate salts immediately outside this narrow range surprisingly produce little or no lather of any note, under normal conditions of use. Evidence to substantiate this fact is given later in this specification. The compositions so obtained are accordingly capable of producing a superior lather volume and an outstanding lather creaminess. Also, the composition is so mild to the skin that it can safely be used for cleansing the mucosae, such as the mouth and the vagina, and other more delicate skin areas. It can also be used in shampoos for frequent, e.g. daily, hair washing, without risk of scalp irritation or damage attributable to harsher products. In addition to these excellent attributes, the ease of rinsing from hair or skin and superior silky-smooth after-use skin feel properties of the compositions, including freedom from skin roughness and erythema, have great consumer appeal.